1. Field of the Invention
The present invention generally relates to wireless communication systems and, more particularly, to a voice activity detector having particular application to mobile radio systems, such a cellular telephone systems and air-to-ground telephony, for the detection of speech in noisy environments.
2. Description of the Prior Art
A voice activity detector (VAD) is used to detect speech for applications in digital speech interpolation (DSI) and noise suppression. Accurate voice activity detection is important to permit reliable detection of speech in a noisy environment and therefore affects system performance and the quality of the received speech. Prior art VAD algorithms which analyze spectral properties of the signal suffer from high computational complexity. Simple VAD algorithms which look at short term time characteristics only in order to detect speech do not work well with high background noise.
There are basically two approaches to detecting voice activity. The first are pattern classifiers which use spectral characteristics that result in high computational complexity. An example of this approach uses five different measurements on the speech segment to be classified. The measured parameters are the zero-crossing rate, the speech energy, the correlation between adjacent speech samples, the first predictor coefficient from a 12-pole linear predictive coding (LPC) analysis, and the energy in the prediction error. This speech segment is assigned to a particular class (i.e., voiced speech, un-voiced speech, or silence) based on a minimum-distance rule obtained under the assumption that the measured parameters are distributed according to the multidimensional Gaussian probability density function.
The second approach examines the time domain characteristics of speech. An example of this approach implements an algorithm that uses a complementary arrangement of the level, envelope slope, and an automatic adaptive zero crossing rate detection feature to provide enhanced noise immunity during periods of high system noise.